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The more I look at that schematic, the more I see room for improvement.
1) I have not taken the time to breadboard the circuit but am not convinced that the Wien Bridge is going to be stable or oscillate at the proper frequency, the component values just don't appear correct. There is no gain control, meaning that the sinewave will be distorted or clipped and the stability will drift with impedence and temperature.
2) I think there should be a buffer stage after the oscillator.
3) An Opamp would likely work much better as an attenuator (instead of the two diodes).
4) I don't see a need for the bridge rectifier. It looks like he is using it in conjunction with an opamp precision rectifier... It does not appear to make sense.
I have a more stable and conventional circuit in the simulator right now. The wien bridge will be equiped with gain control so that it will be self stable. You shouldnt need a scope to set the oscillator then. I have also ordered a sinewave generation chip and will play with that as well.
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